Morph between different facial expressions?
There are at least two ways to create and animate facial expressions, not even involving python (although it's possibly to automate this):


Shape Keys - allows you to deform mesh vertices and blend multiple "states" together and also animate
Armature - a skeleton to control the mesh, with strong control over deformation using vertex weights, but also more complicated


I recommend to try out Shape Keys:


Start Blender and remove (X) the default cube 
Add a Monkey (aka Suzanne)
Go to Object data tab in Properties Editor
Click the plus button + in the Shape Keys panel twice (will create Basis and Key 1)
Switch Suzanne into Edit Mode (Tab)
Deselect all (A) and pick some vertices, e.g. the right eyebrow
Enable Proportional Editing if you wish to deform nearby, non-selected vertices as well
Move the vertices up a bit (G)
Go back to Object Mode (Tab)
With Key 1 in the Shape Keys panel selected, move the Value slider to see the transition between the base shape and the keyed shape.
Use I over the slider or right-click and pick Insert Keyframe to animate the value and thus the facial expression transition!


You can create a Shape Key with Python like:

shapekey = bpy.context.object.shape_key_add(name="Key", from_mix=True)


See the API docs - Object.shape_key_add for details.

The value slider can be animated by inserting a keyframe, here's the most simple way:

# we stored the shape key reference above in the variable "shapekey",
# which should be equal to bpy.context.object.data.shape_keys.key_blocks['Key']
shapekey.value = 0.3 # set value 
shapekey.keyframe_insert(data_path="value", frame=15) # now keyframe 0.3 at frame 15


See the API docs - keyframe_insert()



More on Shape Keys:http://wiki.blender.org/index.php/Doc:2.6/Manual/Animation/Techs/Shape/Shape_Keys

If you look into Armature pose animations, make sure to have a look at the Pose Library panel!

It's also worth to have a look at the NLA editor (non-linear animation). It allows you to blend multiple Actions together:http://wiki.blender.org/index.php/Doc:2.6/Manual/Animation/Editors/NLA